Discharge device



Feb. 11, l1. L. D. MILES DISCHARGE DEVICE Filed July 26, 1938 Inventor: Lawrence D. Miles,

bg WLM/17 His Attorney Patented Feb. 11, 1941 UNITED STATES DISCHARGE DEVICE Lawrence D. Miles, Ballston Lake, N. Y., assigner to General Electric Company, a corporation of New York Application July 26, 1938, Serial No. 221,327

4 Claims.

The novel features which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself may best be understood by reference to the following description taken in connection with the drawing in which Fig. l comprises a longitudinal sectional view of a discharge device suitably embodying my invention and Fig. 2 is an enlargement of the make-alive electrode of Fig. 1.

Referring particularly to Fig. 1, there is shown an elongated envelope comprising a metal cylinder I0 and a pair of headers II and I2 closing the ends of the cylinder. The bottom header is reinforced with a backing plate I3 to which is secured a terminal lug I4. The upper header includes a plurality of lead-in connections for the various enclosed electrodes.

Within the envelope and in contact with the wall-s thereof there is provided a quantity of cathode material I6. In the case contemplated by the present invention this is a material which is solid at ordinary temperatures (i. e. room .temperature) and which attains a substantial vapor pressure only at relatively high temperatures. A preferred example of such a material is cadmium although other substances, for instance, tin, may be alternatably employed. It is the function of the cathode material to provide a source of electrons and to provide an ionizable vapor for reducing the voltage drop through the device during operation. It may be supplemented in the latter respect, if desired, by the provision in the discharge space of a xed gas, such as argon, at an appropriate pressure.

Cadmium is considered a desirable substance for the use specified because its vapor pressure is not excessive at temperatures as high as 200 to 250 C. At these temperatures even a high capacity device can be operated satisfactorily with air-cooling only. That is to say, the high temperature gradient which exists between the device and the ambient air permits a substantial amount of heat to be removed from the device without the assistance of a liquid cooling system such as is ordinarily required With large mercury pool tubes.

S'paced from the cathode and cooperating with it there is provided an anode I8, suitably consisting of graphite. This in insulatingly supported from the header II by means of a glass-to-metal seal which includes a glass ring 20 and metal ele- `ments 2I and 22 hermetically joined to the ring.

(Cl. Z50-27.5)

A heavy shaft 24 serves to mount the anode from the element 2 I', and an outwardly extending metal cylinder 25 facilitates the connection of the anode to a source of potential. A metal sleeve 26 protects the lower glass-to-metal joint from the high potential gradients existing in the vicinity of the anode.

It will be noted that the cathode material I6 is shown as extending a substantial distance up the wall of the enclosing envelope. This disposition of the cathode material is a result of the tendency of the material to condense on the relatively cool walls of the cylinder I0. It is in a sense objectionable in that it makes it readily possi-ble for the cathode spot to leave the surface of the cathode and to .travel up the envelope into the region of the electrode seals. In order to prevent such operation there is provided adjacent to the top of the envelope an annular sleeve 30 which is concentric with the walls of the envelope and is slightly spaced therefrom. The narrowly constrcted space which exists between this sleeve and the envelope has the effect of preventing the cathode spot from running substantially above the highest point reached by the cathode material. In particular, it prevents the cathode spot from reaching the anode seals or the other seals associated with the header II.

It is desirable to avoid condensation of cathode material on the anode and especially on the insulating parts which are associated with the upper portion of the envelope. This may be done by the provision of means for assuring that the temperature of the parts desired to be protected shall be above the temperature maintained in the lower portions of the device. One expedient which may be resorted to in this connection comprises the use of a heating element in connection with the anode structure. Such an element is indicated generally at 32 where there is shown a resistance heater of known type embedded in an insulating body 33. Lead-in connections 34 and 35 are provided for supplying current to the heater. Maintenance of a relatively high temperature in the anode section of the envelope is further assured by means of a layer of thermal insulation 3l applied to the outside of the envelope. Such linsulation may consist, for example, of asbestos or the like.

In order to render the discharge device initially operative there is provided a make-alive electrode 38. This comprises a body of semi-conducting material which `has a specic resistivity between about 10"2 and 105 ohm centimeters such, for example, as silicon carbide, and which projects partly above and partly below the surface of the cathode material. Upon passage of a starting current, this electrode functions by virtue of its high resistivity to produce electron emission from the surface of the cathode and thereby to initiate a cathode spot. Starting current may be supplied to the electrode 38 through a lead-in conductor 39 which is supported from the upper header of the enclosure by means of a glass-to-metal seal All. A layer of insulating material 1H, for example, alumina, is applied to the conductor 39 to protect it from the eects of the discharge.

After the cathode spot is initiated, ionization may be maintained in the discharge space by means of auxiliary electrodes 43 and M which function to produce a holding arc to the cathode surface. Once such a holding arc is established, the electrode 38 may be deenergized.

Due to the location of the electrode 38 at the surface of the cathode there is a considerable tendency for the cathode material to be condensed on it. Such material may form a conductive coating over the electrode which necessarily decreases its resistance and may prevent its operating in the desired fashion when it is desired to start a discharge. In order to overcome this difculty my invention provides in connection with the electrode 38 a heating element operative during initial starting periods to vapo-rize Whatever cathode material may be condensed on the electrode surface. This heating element is shown in the present case as comprisi-ng a lamentary heater 46 arranged in good heat-exchanging relation with the electrode 3S. It may consist, for example, of a tungsten wire which is coiled into spiral form as shown. With the heating element 48 in place, if the device fails to start upon passage of normal starting current through the electrode 38, the difculty may be overcome simply by continuing the application of current long enough to raise the electrode to a temperature above the melting temperature of the cathode material or at least above the temperature of the neighboring parts of the tube. In this way the condensed material may be vdriven off the electrode surface. The further application of current, therefore, acts in the normal way to render the discharge device conductive.

A further problem which is encountered inthe construction of a device, such as that shown, has to do with the provision of a suitable mounting for the electrode 38. The semi-conducting materials (e, g. silicon carbide or the like) from which such electrodes are ordinarily constructed are characterized by a. high degree of fragility and brittleness. Since in the arrangement shown, the electrode 38is in direct Contact with a solid unyielding body of cathode material, it will beseen that itV will be subjected to considerable stress as a result of relative expansive movements of the envelope parts. More specifically, the electrode 33 or its mounting seal may be broken because the envelope l() expands to a degree which is unequal to the expansion of the cathode. material and the electrode mounting.

In order to obviate this danger there is provided a flexible element forming a part of the mounting of the electrode 33. Preferably, this element is identical with the filament which serves as a heater for the electrode. This is readily possible, for example, if the heater is composed of ductile tungsten which has been iired at high temperature in hydrogen to give it a resilient character. The lament may be secured to the upper part of the electrode 38 by means of a clamping connection 41 and to the lower end ofthe conductor 3Q. by being welded directly thereto. With this arrangement any unequal expansion of the associated parts will be taken up in the coiled lament 46 and little or no stress will be imposed on the electrode 38.

While I have described my invention in connection with a particular embodiment thereof it k will be understood that numerous modifications may be made by those skilled in the art without departing from the invention. I, therefore, aim in the appended claims to cover all such equivaient variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A discharge device including an anode, a cathode which is in the solid state at some normally encountered condition of starting or operation, a make-alive electrode projecting partly above and partly below the surface of the cathode material, said electrode being constituted of a substance which has a specific resistivity between about lxz and about 105 ohm centimeters, whereby pasage of current through the electrode establishes potential gradients along the electrede surface sulicient to initiate electron emission from the cathode material, and heating means associated with the make-alive electrode for removing cathode material which condenses on the exposed portion of the electrode during inoperative periods of the device.

A Idischarge device including an anode, a cathode comprising a mass of solid cathode material, a make-alive electrode projecting partly above and partly below the surface of the cathode material, said electrode being constituted of a substance which has a specific resistivity between` about -2 and about 105 ohm centimeters, whereby passage of current through the electrode establishes potential gradients along the electrode surface suflicient to initiate electron emission from the cathode material, and a filamentary heater in good heat-exchanging relation with the exposed portion of the make-alive electrode, said heater being effective on starting to remove cathode material which oondenses on the exposed portion of the make-alive electrode during periods of disuse of the device.

3. A discharge device including an envelope which encloses an anode, a cathode comprising a mass of solid cathode material, a make-alive electrode in permanent contact with the cathode material and having a specific resistivity between about 1G2 and about 105 ohm centimeters whereby passage of current through the electrode establishes potential gradients along the electrode surface suliicient to initiate electron emission from the cathode material, said make-alive electrode being composed of a semi-conducting substance of the class represented by silicon carbide and being of relatively fragile character by virtue of such composition, means for supporting the make-alivey electrode from the envelope, and a readily flexible element forming a part of the said supporting means whereby relative expansive movements ofthe parts of the discharge device areprevented-from fracturing the electrode.

4; A discharge device according to claim 1 wherein the heating. means comprises a resistance lament which, in addition to its heating function, serves to support the make-alive electrode resiliently from the envelope, thereby to prevent relative expansive movements of the parts of the device yfromY exerting excessive pressure on the make-alive electrode.

LAWRENCE D. MILES; 

